1. Field of the Invention
The present invention relates to acoustic wave devices and filters, and more particularly, to an acoustic wave filter having a high controllability of the electromechanical coupling constant and a filter using the same.
2. Description of the Related Art
An exemplary acoustic wave device that utilizes an acoustic wave is a surface acoustic wave (SAW) device equipped with a piezoelectric substrate on which there are formed comb electrodes that form an interdigital transducer (IDT) and reflection electrodes. When electric power is applied across the comb electrodes, a surface acoustic wave is excited. Hereinafter, such a SAW device is referred to as a first prior art. The SAW device advantageously has a compact size, light weight and a high attenuation, and is widely applied to a variety of mobile equipment such as transmission/reception filters and an antenna duplexer of a cellular phone.
FIG. 1A is a plan view of a SAW device of the first prior art, and FIG. 1B is a cross-sectional view taken along a line A-A shown in FIG. 1A. The SAW device has a piezoelectric substrate made of, for example, lithium tantalate (LiTaO3) on which comb electrodes 14 and reflection electrodes 16, which electrodes may be formed of aluminum, are provided. For the sake of simplicity, only a small number of fingers of the comb electrodes 14 and the reflection electrodes are illustrated in FIG. 1A. However, in practice, a large number of fingers is used to form the electrodes 14 and 16.
A variation of the first prior art has been developed (hereinafter such a variation is referred to as second prior art). As shown in FIG. 2, a protection form 18 made of silicon oxide (SiO2) covers the comb electrodes 14 and the reflection electrodes 16. The protection film 18 has a thickness as thin as only a few percent of the period λ of the comb electrodes 14.
Another variation of the first prior art has been developed. This variation also has the protection film 18. However, the present variation differs from the second prior art in that the protection film 18 is thicker than the comb electrodes 14. Examples of this type of acoustic wave devices are a Love wave device (hereinafter referred to as a third prior art) and a boundary wave (fourth prior art). An acoustic wave called Love wave is propagated in the Love wave device. An acoustic wave called boundary wave is propagated in the boundary wave device. In the Love wave device and the boundary wave devices, the protection film 18 may be made of a substance having a temperature coefficient of a sign opposite to that of the temperature coefficient of the substrate of the piezoelectric substrate 12. It is thus possible to compensate for the temperature characteristic of frequency (frequently abbreviated as TCF). Particularly, the boundary wave device has an advantage that foreign particle at the interface between the two media does not cause frequency variation and does not increase the electrical loss.
FIG. 3 is a cross-sectional view of a Love wave device of the third prior art. Referring to this figure, there are provided the comb electrodes 14 and the reflection electrodes 16 on the piezoelectric substrate 12. A silicon oxide film 20 is provided so as to cover the comb electrodes 14 and the reflection electrodes 16.
FIG. 4 is a cross-sectional view of a boundary wave device of the fourth prior art. Referring to this figure, the boundary wave device differs from the Love wave device in which an aluminum oxide (Al2O3) film 22 is provided on the silicon oxide film 20 of the Love wave device of the third prior art. The boundary wave device has an essential feature such that energy of the acoustic wave is confined in the surface of the piezoelectric substrate 12 and the silicon oxide film 20.
In the acoustic wave devices described above, the interval between the resonance frequency and the anti-resonance frequency depends on the electromechanical coupling coefficient. Thus, the width of the pass band of a ladder filter and a double-mode filter using the above acoustic wave devices depends on the electromechanical coupling coefficient. It is thus necessary to make the piezoelectric substrate of a piezoelectric material having an electromechanical coupling coefficient that matches a desired pass band. However, various types of piezoelectric material are not available in practice. In view of the above circumstance, there have been proposed various methods for controlling the electromechanical coupling coefficient. The electromechanical coupling coefficient is the conversion efficiency from electrical energy to piezoelectric or acoustic energy. As the electromechanical coupling coefficient is greater, the acoustic wave by the electric signal is excited more easily.
Japanese Patent Application Publication No. 52-16146 (hereinafter Document 1) discloses the use of a titanium oxide (TiO2) film between the comb electrodes and the piezoelectric substrate. Thickening the titanium oxide film can reduce the electromechanical coupling coefficient. Japanese Patent Application Publication No. 6-303073 (Document 2) discloses a method for controlling the piezoelectricity by implanting ions such as argon (Ar) on the surface of the piezoelectric substrate. This ion implantation reduces the electromechanical coupling coefficient. Japanese Patent Application Publication No. 11-31942 (document 3) discloses a technique to use a piezoelectric thin film that is provided between the comb electrodes and the piezoelectric substrate and has a greater electromechanical coupling coefficient than that of the piezoelectric substrate.
The method for controlling the electromechanical coupling coefficient disclosed in Document 1 uses a specific thickness of the titanium oxide film that ranges from 0.00016λ to 0.047λ. However, the above specific thickness is too thin to form filters for cellular phones in practice. For example, the acoustic wave device used for the cellular phones generally employs a piezoelectric substrate made of 36° Y-cut X-propagation lithium tantalate. When this piezoelectric substrate is used to form a filter in the 1.9 GHz band, the comb electrodes have a period λ of 2 μm. For this period, 0.00016λ is equal to 0.32 nm, which is too thin to practically form the electrodes in terms of the fabrication process. In addition, the electromechanical coupling coefficient is changed greatly as the thickness of the titanium oxide film is changed. It is thus difficult to control the electromechanical coupling coefficient well.
The method for controlling the electromechanical coupling coefficient described in Document 2 requires a large scale of facility such as ion implantation. In addition, there is a difficulty in control of the depth of ion implantation. For these reasons, the controllability of the electromechanical coupling coefficient is not good.
The control method described in Document 3 has a difficulty in controlling of the thickness of the piezoelectric thin film.
In short, all of the methods described in Documents 1 through 3 have difficulty in controlling the electromechanical coupling coefficient.